Low-crystalline ethylene copolymers such as ethylene-.alpha.-olefin copolymers have been heretofore used widely as modifiers for thermoplastic resins such as polyethylene, polypropylene and an ethylene/vinyl acetate copolymer.
Polyolefins including these ethylene copolymers have been conventionally prepared by a solution polymerization process or the like using a titanium type catalyst formed from a titanium compound and an organoaluminum compound or using a vanadium type catalyst formed from a vanadium compound and an organoaluminum compound.
Recently, a novel Ziegler type catalyst for olefin polymerization formed from a transition metal compound such as zirconocene and an organoaluminum oxy-compound has been proposed as the catalyst for olefin polymerization, and it is known that this Ziegler type catalyst is able to polymerize or copolymerize olefin with high activities and to prepare olefin copolymers which are excellent in composition distribution (i.e., olefin copolymers having narrow composition distribution).
The organoaluminum oxy-compound mentioned as above is generally prepared by bringing an organoaluminum compound such as trialkylaluminum into contact with hydrate of metal salt in a hydrocarbon solvent. In this case, aromatic hydrocarbon which well dissolves the resulting organoaluminum oxy-compound, especially toluene, is used as the hydrocarbon solvent. If the organoaluminum compound is brought into contact with the hydrate of metal salt in an aliphatic hydrocarbon solvent such as hexane, the resulting organoaluminum oxy-compound is difficultly separated from the metal salt because the organoaluminum oxy-compound is hardly dissolved in the aliphatic hydrocarbon solvent.
Accordingly, the organoaluminum oxy-compound has been conventionally prepared in the form of a solution containing said organoaluminum oxy-compound in aromatic hydrocarbon such as toluene, that is, in the form of a toluene solution of the organoaluminum oxy-compound, and the solution has been added to the polymerization system.
By the way, when polyolefin is prepared by polymerizing or copolymerizing olefin in a liquid phase in accordance with a solution polymerization process, the resulting polymer (i.e., polyolefin) is obtained in the form of a polymer solution wherein polyolefin is dissolved in a solvent, and therefore it is necessary to separate the polyolefin from the solution to obtain the polyolefin.
If toluene is used as a solvent for polymerization as described above, there is involved such a problem that the drying step of the above-mentioned polymer solution under heating or at a reduced pressure to dry the resulting polymer can be hardly simplified because toluene has a high boiling point. Further, there is other problem that toluene has an offensive odor and the odor of toluene tends to remain in the resulting polymer.
It can be thought that benzene is used instead of toluene, but the use of benzene is unfavorable in view of industrial hygiene.
Accordingly, there has been eagerly desired the advent of such a process for preparing polyolefin that when polyolefin is prepared by solution polymerization or the like in the presence of a hydrocarbon solvent, the drying step of the resulting polymer can be simplified, and that polyolefin having no odor of the solvent for polymerization can be easily prepared.
For efficiently separating and recovering the resulting polyolefin from the aforesaid polymer solution, Japanese Patent Laid-Open Publication No. 58(1983)-7402 discloses a polymerization process which is characterized in that the polymer solution is fed to a separation zone to separate the polymer solution into a lower phase portion containing the resulting copolymer in a high concentration and an upper phase portion containing the resulting copolymer in a low concentration, then the copolymer is recovered from the lower phase portion, and at least a part of the upper phase portion is recycled to the polymerization reaction. Further, Japanese Patent Laid-Open Publication No. 59(1984)-206416 discloses a process for preparing polybutene-1, which is characterized in that the polymer solution is fed to a separation zone kept at a temperature of not lower than the upper cloud point of the polymer solution to separate the polymer solution into a lower phase portion and an upper phase portion, then the resulting polybutene-1 is recovered from the lower phase portion in the separation zone, and at least a part of the upper phase portion is recycled to a polymerization zone.
In each of the processes for preparing polyolefin described in the above publications, a highly active catalyst composed of a solid titanium catalyst component and an organoaluminum compound is used as a catalyst, and according to those processes, separation of the resulting polymer from the polymer solution can be efficiently carried out, and moreover polyolefin such as a low-crystalline ethylene copolymer can be efficiently prepared.
Though the polyolefin obtained using the above-mentioned highly active catalyst is excellent in physical properties, there has been further desired the advent of such a process as capable of preparing polyolefin (e.g., low-crystalline ethylene copolymer) showing much more improved physical properties, for example, higher transparency and smaller change of heat-sealing strength with time.
In the light of the prior art as mentioned above, the present inventors have earnestly studied on the process for preparing polyolefin by polymerizing or copolymerizing olefin in a liquid phase, and-as a result, they have found that the drying step can be simplified and polyolefin having no odor of a solvent for polymerization can be easily obtained by feeding a specific catalyst for olefin polymerization to the polymerization system in a specific manner and polymerizing or copolymerizing olefin in the presence of the specific catalyst.
The present inventors have also found that polyolefin which is excellent in various physical properties can be efficiently prepared through simplified steps by a process comprising polymerizing or copolymerizing olefin in a liquid phase in the presence of the above-mentioned specific catalyst for olefin polymerization to prepare a polymer solution, then separating the polymer solution into a lower phase portion and an upper phase portion in a separation zone kept at a temperature of not lower than the upper cloud point of the polymer solution, and recovering the resulting polyolefin from the lower phase portion while recycling at least a part of the upper phase portion to a polymerization zone. Thus, the present invention has been accomplished.